currently have a lvl1 vault.

will my items remain safe during upgrade? should i move them out?

when i upgrade to say lvl3, will it subtract the cost for my lvl1? Or should I save till I can get something bigger and stick with that?

Are you using the /vaults? With the vaults purchased from /vaults, you don't upgrade. Whenever you buy a new vault, the old one remains. The new one you buy you get to by typing /vault 2.

I'm not sure, but I don't think you can upgrade from a 1smallvault to a 3largevault. When you upgrade a vault, I'm fairly certain they just knock down some wall and add a chest to two. So, your items will be safe.

ooo, that's cool

then i can just buy some cheap ones for the storage space.

If thats what you want to do, go for it! Just good luck keeping track with the lot (of vaults that is) xD